A type of dissociated misfit dislocation in epitaxial Ba0.3Sr0.7TiO3 thin films on (001) LaAlO3 substrates has been studied by high-resolution transmission electron microscopy. The dislocation has a Burgers vector [210] and is dissociated into four partial dislocations with Burgers vectors of type 1 2k110l. All partials can relieve the localized misfit strain and they are interacted with three 1 2k110l stacking faults lying on (001). The partials were generated during island nucleation and mosaic growth of the BSTO film, while a small amount of excess TiO2 during film deposition favored the formation of the 1 2k110l stacking faults.